1. Field of the Invention
The present invention relates to a servo circuit and, more particularly, to a means for controlling a variable gain amplifier to process an audio signal in a television receiver, a stereo apparatus or the like.
2. Description of the Prior Art
There is known a servo circuit for controlling the gain in a signal, which is to be processed, by changing a control voltage or current supplied to a variable gain amplifier (hereinafter referred to as VCA) to vary the gain of such amplifier. Servo circuits are classified into a closed loop type and an open loop type, and a variety of methods and circuit configurations are presently contrived for controlling a VCA. However, in forming such a servo circuit into an IC (integrated circuit) configuration, there exist some disadvantages due to nonuniformity of the temperature characteristic or specific characteristics inherent in a bipolar-transistor IC. According to the known methods, a temperature characteristic canceler is inserted in a VCA control path to improve the temperature characteristic of an entire servo circuit, or a switch means is inserted for halting the VCA gain variation in a certain state so as not to cause any gain change in the VCA despite any level change of the control signal level.
In the conventional servo circuit, if the VCA control characteristics or the upper and lower limits of the control voltage or current have a temperature characteristic, it follows that the gain control range of the servo circuit as a whole will have a temperature characteristic.
FIG. 1 is an exemplary block diagram of a conventional closed-loop servo circuit. In this diagram, a process signal S1 fed to a process signal input terminal T1 is supplied to a VCA 1, and a control signal S2 fed to a control signal input terminal T2 is supplied to a first level detector 2. Meanwhile a VCA output signal S3 is delivered to an output terminal T3 and then is supplied to a second level detector 3. Output signals S4 and S5 of such first and second level detectors 2, 3 are added to each other by an adder 5 with the polarities thereof rendered mutually inverse. An output S6 of the adder 5 is delivered to a controller 4, which produces a control signal S7 for controlling the gain of the VCA 1 in accordance with the polarity of the adder output 6 and then delivers the signal S7 to a limiter 6. For example, when the level detection outputs S4 and S5 of the polarities shown are supplied to the adder 5, a control signal S7 for increasing the gain of the VCA 1 is outputted from the controller 4 in the case where the adder output S6 is positive, whereby the gain of the VCA 1 is increased so that, of the two signals supplied to the adder 5, the negative signal S5 is gradually rendered greater, and the servo action is completed upon arrival of the adder output S6 at zero.
The limiter 6 is supplied with an upper limit setting signal S8 from an upper limit generator 7 and also with a lower limit setting signal S9 from a lower limit generator 8, and serves to change its output signal S10 between the upper and lower limit values.
The output signal S10 from the limiter 6 is supplied to a temperature characteristic canceler 9 provided for canceling the temperature characteristic included in the control characteristics of the VCA 1. The signal S10 obtained from the limiter 6 is outputted as a gain control signal S11 from the temperature characteristic canceler 9 and then is delivered to the VCA 1.
In addition to FIG. 1 representing an exemplary case where the servo circuit is formed into a closed loop, a similar operation is performed also in the case of FIG. 2 where the servo circuit is formed into an open loop.
In the conventional servo circuit of the configuration mentioned, it is necessary to insert a canceler 9 for canceling the temperature characteristic included in the control characteristics of the VCA 1 as described, hence enlarging the circuit scale while increasing the power consumption and the production cost as well.
Since it is not desired that the signals S8 and S9 outputted respectively from the upper limit generator 7 and the lower limit generator 8 have a temperature characteristic, a constant voltage circuit (or constant current circuit) is required to cancel the temperature characteristic of the upper limit setting signal S8 and the lower limit setting signal S9. However, the provision of such a canceler circuit renders the entire servo circuit configuration further complicated. Although the circuit configuration may be simplified by the use of a limiter 6 with a diode, zener diode or similar element, any limiter with such an element has a temperature characteristic and therefore some disadvantages are induced inclusive of nonuniform operation and characteristic variations due to temperature fluctuations.
Now a description will be given on another conventional example where gain variation is reduced by the use of a switch circuit in a feedback path of a closed-loop servo circuit.
As shown in a block diagram of FIG. 3, there is known a servo circuit for controlling the gain of a VCA by an output voltage of an integrator. In such a servo circuit, both the level of an input signal to be processed and the level of a control signal are detected and calculated, and then the voltage held by the integrator is changed in accordance with the result of such calculation to change the output voltage, thereby controlling the gain of the VCA.
During such gain control action, it is occasionally needed to execute an operation of halting the VCA gain variation so as not to cause any gain change in the VCA despite any change of the control signal level. Such gain control can be achieved by interrupting the current flowing in the integrator. More specifically, when the input current to the integrator is interrupted, the integrator holds the voltage at such instant, so that the control voltage for the VCA is maintained at a fixed value to thereby retain the gain of the VCA at a certain fixed value.
FIG. 3 is a block diagram of an exemplary servo circuit based on the prior art. In this diagram, there are included a VCA 31, a first level detector 32, a second level detector 33, a calculator 34, an integrator 35, and a switch 30. The switch 30 is provided for interrupting an input current flowing into the integrator 35. It has been generally customary in the conventional servo circuit to employ a MOS FET or a junction FET as the switch 30.
Therefore, in manufacturing the servo circuit of FIG. 3 by a bipolar production process, an additional step of forming a FET for the switch 30 is required which consequently raises the production cost. For the purpose of averting such increase in the production cost, there may be contrived an improvement where, as shown in a circuit diagram of FIG. 4, a bipolar transistor Q1 is interposed between the calculator 34 and the integrator 35 to constitute the switch 30. In this case, it becomes possible to eliminate the necessity of adding the FET forming process for the switch 30, hence solving the problem of an increase in the production cost that is derived from the additional process step. However, there occurs an operational inconvenience as will be described below. In an exemplary circuit configuration where the output of a calculator is provided in the form of a voltage, if the voltages at individual portions are such as those shown in FIG. 4, the output current I.sub.i obtained in the absence of a switch 36 is expressed as EQU Input current I.sub.i =(V1-V2) / R1 (1)
Therefore the input current I.sub.i comes to have a value determined at the input terminal of the integrator 35.
When the bipolar transistor Q1 in the circuit of FIG. 4 is turned on, the input current I.sub.i comes to flow in the resistor R1 in accordance with the potential difference across the resistor R1. It is expressed as EQU Input current I.sub.i =(V1-V3) / R1 (2)
The voltage V3 is rendered lower than the control voltage V4 by a value corresponding to the base-emitter voltage V.sub.BE. However, since the base-emitter voltage V.sub.BE changes depending on the input current I.sub.i or the temperature, it follows that the voltage V3 also varies correspondingly thereto. Accordingly the input current I.sub.i changes in conformity with the temperature fluctuation and so forth, and the current inputted practically to the integrator 35 fails to be accurately proportional to the output voltage V1 of the calculator 34, hence inducing a disadvantage in that the precision is deteriorated when the bipolar transistor Q1 is used as the switch 30.